To get another user's attention, mention them – type @ and then the first part of their name. Your mention will be highlighted on their screen – @someone. This will happen even if they are in a different room. Note that you can only mention @someone who has been in the room at some point. If you need to invite someone to your room, do it from the user's chat profile page or from the user card that pops up when you click on their avatar in a chat room.
Most of us oldtimers know the window of pingability is about a week. There's a meta post that talks about it - and while its community documented, it would be nice to have how recently you need to be on chat last to be pinged less vaguely included organically in the help pages, for better retrievability. Could we have the help pages updated to make this clearer?